The development of optical memories on/from which information is optically recorded, reproduced or erased is in progress. An optical memory is a recording medium formed by a substrate in the shape of, for example, a disk or a card, covered with a recording material film. In the case of a disk-shaped substrate, spiral or concentric grooves are preformed thereon. The grooves and lands between the grooves form tracks. During recording or reproduction of information, a light spot projected onto an optical memory is controlled to follow the track.
The substrate of the optical memory is produced with the use of a stamper 25 manufactured through the processes shown in FIGS. 8(a) through 8(g). Firstly, as illustrated in FIG. 8(b), a photoresist 21 is applied to the surface of a substrate 20 shown in FIG. 8(a). Secondly, as illustrated in FIG. 8(c), argon laser light 22 is projected onto predetermined portions of the photoresist 21 so as to record tracks. Subsequently, as shown in FIG. 8(d), the photoresist 21 is developed. Next, a nickel film 23 is formed thereon by, for example, sputtering (see FIG. 8(e)), and a nickel layer 24 is then electroformed on the nickel film 23 (see FIG. 8(f)). Finally, the nickel layer 24 is peeled off from the substrate, whereby the stamper 25 on which the track structure is transferred is obtained as illustrated in FIG. 8(g). The substrate of the optical memory is manufactured by injection molding using the stamper 25.
Next, with reference to FIG. 9, the following discusses the structure of an optical pickup. The optical pickup forms a light spot by converging light and directs the light spot to follow the track on an optical disk 31 as an optical memory. As for the optical disk 31, a recording material film 39 is formed on the surface of a substrate 30 manufactured by using the stamper 25.
Light emitted by a semiconductor laser 26 as a light source passes through a shaping prism 27 and a first half prism 28, and is then converged on the recording material film 39 of the optical disk 31 by an objective lens 29. Reflected light from the optical disk 31 is reflected by the first half prism 28 and falls onto a second half prism 32. After falling onto the second half prism 32, the light is separated into two, namely, light directed to a spot lens 36 and light directed to a polarizing beam splitter 33.
The light incident onto the spot lens 36 is directed to pass through a cylindrical lens 37 and is then received by a 4-quadrant photodetector 38. By detecting the difference between the output signals from the left and right halves of the 4-quadrant photodetector 38, a tracking error signal is obtained. The tracking error signal indicates deviation of the light spot from the track center.
Since the light spot is diffracted by the track when traversing the track, the amount of reflected light changes. A track crossing signal is derived from the sum of the output signals from the four detection sections of the 4-quadrant photodetector 38. Namely, the track crossing signal indicates changes in the amount of reflected light when the light spot traverses tracks one after another. When the optical pickup moves to access to a target track, the number of tracks traversed by the light spot is detected by counting the number of times the waveform of the track crossing signal reaches its positive peak value. Then, the optical pickup is positioned according to the detected number.
Meanwhile, the light incident onto the polarizing beam splitter 33 is further separated into two and received by photodetectors 34 and 35, respectively. Accordingly, various other signals are generated.
FIGS. 10(a) and 10(b) illustrate a relation between a push-pull signal (tracking error signal) and a track crossing signal. FIGS. 10(a) and 10(b) indicate the push-pull signal and the track crossing signal produced when the optical pickup moves in a direction and crosses the track, respectively.
There is a phase difference of 90 degrees between the track crossing signal and the push-pull signal, and the phase of the track crossing signal is delayed by 90 degrees with respect to that of the push-pull signal as shown in FIGS. 10(a) and 10(b). The phase difference between the track crossing signal and the push-pull signal varies depending on the moving direction of the optical pickup. If the optical pickup moves in the opposite direction, the phase relation between the track crossing signal and the push-pull signal is inverted and the track crossing signal advances by 90 degrees with respect to the push-pull signal. Thus, the moving direction of the optical pickup is detected by detecting the phase relation between the track crossing signal and the push-pull signal. This makes it easier for the optical pickup to access to a target track.
The track crossing signal and tracking error signal thus obtained vary greatly depending on track parameters, such as the width and depth of grooves forming the tracks and the track pitch. Since the C/N of the signals improves when the reflectance at the track increases, it is desirable to make the reflectance as high as possible.
Moreover, to enhance the recording capacity of the optical memory, recording density must be increased. One of the effective methods to increase the recording density of the optical memory is that increasing the recording density along the track direction while decreasing the track pitch. However, since the track parameters change when the track pitch is decreased, the respective signals derived from the tracks vary significantly. Thus, in order to obtain appropriate signals and reflectance, the track parameters must be designed carefully.
As shown in FIG. 11, the intensity of the track crossing signals was measured with respect to various groove depths and widths. These measurements were carried out using optical memories with a track pitch of about 1.6 .mu.m, an objective lens with a numerical aperture (NA) of 0.55, and laser light with an 830-nm wavelength. The intensity of the track crossing signal is normalized on the basis of the intensity of light reflected from a non-groove portion.
It can be seen from FIG. 11 that the intensity of the track crossing signal becomes maximal when the width of a groove is 0.3 .mu.m to 0.4 .mu.m. Moreover, in the case where the depth of a groove is in the range of 70 nm to 100 nm, the intensity becomes greater as the depth of the groove becomes greater. The sufficient intensity of the track crossing signal varies depending on each system. A groove depth of about 70 nm or more is required in order to obtain a sufficient intensity of, for example, about 0.2 in each system. Thus, when the track pitch is about 1.6 .mu.m, the appropriate width and depth of the groove are about 0.35 .mu.m and 70 nm, respectively.
However, if grooves are formed with the above-mentioned dimensions but with a smaller track pitch, the results shown in FIG. 11 are unlikely expected.
The following document discusses track parameters in detail. The title of the document is "Designing preformed grooves and preformed pits of optimum dimensions for push-pull/tracking servo system", Optical Memory Symposium '90, p. 11. However, this document describes only the track parameters when the track pitch is 1.6 .mu.m and does not mention designing of track parameters with respect to a smaller track pitch.
The dimensions of tracks are described in the following documents. "Optical pregroove dimensions: design considerations", Applied Optics, Vol. 25, No. 22, Nov. 15, 1985, p. 4031; Japanese Publication for Unexamined Patent Application No. 100248/1983, No. 102347/1983, No. 102338/1983, No. 38943/1984, No. 38944/1984 and No. 11551/1984; and Japanese Publication for Unexamined Utility Model Application No. 165794/1983. However, these documents do not discuss track pitch.
The cases where a smaller track pitch is used are described in the following documents. "Magneto-optical disk by contact printing method", SPIE Vol. 1078, Optical Data Storage Topical Meeting, 1989, p. 204; and "High-density magneto-optical disk using a glass substrate", Papers Presented at Kansai District Regular Science Lecture Meeting of Society of Precision Optics, 1988, p. 107. However, since optimization of track parameters was not performed according to those documents, some problems arise. Namely, the access operation can not be performed with accuracy, because the level of the track crossing signal is lowered when the track pitch becomes smaller.
Next, problems associated with the manufacturing methods of the stamper 25 of FIG. 8 are discussed below. The light spot of the argon laser light 22, which is used when manufacturing the stamper 25, normally has Gaussian distribution or a similar intensity distribution. Namely, a distribution where the light intensity continuously decreases from the center of the light spot outward and it decreases gradually, in particular, at the circumferential section of the light spot.
Therefore, when recording is performed with the argon laser light 22 having such an intensity distribution, the developed photoresist 21 has curved edges. As a result, when substrates for optical memories are produced through injection molding using such a stamper 25, the edges of the lands on these substrates are curved because of the intensity distribution of the argon laser light 22.
Thus, during recording or reproducing of information, when the optical pickup projects a light spot onto the optical memory whose lands have narrowed flat portions due to the curved land edges, the reflectance at the tracks on the optical memory decreases. Consequently, the quality of the signals deteriorates.